1. Field of the Invention
This invention pertains generally to measuring flow characteristics of viscous materials, and more particularly to non-invasive measurement of rheological information using nuclear magnetic resonance spectroscopy and imaging.
2. Description of the Background Art
Many important technologies involve the processing of non-Newtonian fluid materials which have difficult to characterize flow or rheological properties. Thermoplastics are a class of industrially important non-Newtonian fluids which exhibit a full range of theological properties including a steady shear viscosity, .eta., (or shear stress, .sigma.) that depends upon shear rate, .gamma., dynamic viscoelastic properties, normal stresses and time dependent properties. Many slurries found in process industries, such as food and pulp slurries, also exhibit such complex rheology. Other effects, such as phase separation, may also be present due to the multiphase nature of the suspension.
The macroscopic rheology directly reflects the microstructure of the materials being processed. For a thermoplastic, the molecular weight, degree of polymer branching and molecular configuration all play important roles. Properties of suspensions are principally dominated by the particle concentration, size and morphology. Hence, the macrorheological properties are a significant indicator of a wide variety of factors to be monitored and controlled during processing. Further, these properties, particularly the shear viscosity, directly affects the performance of extruders, pumps and other processing equipment. Achieving a particular production rate and product quality requires knowing the viscosity so that the proper motor speeds, heating rates and other process variables can be specified. Therefore, the ability to monitor the viscosity is essential for control of unit operations and assuring product quality.
Process monitoring rheometers are generally categorized either as on-line or in-line devices. On-line rheometers monitor the properties of a side stream of material, whereas in-line rheometers monitor the properties at a point in the process itself. The actual rheological property that is measured varies; however, most instruments measure the steady shear viscosity. The principle is generally the same--a pressure drop is measured during flow through a slit or die. Also, in most, the viscosity is determined at a single shear rate (i.e., a "single point" measurement). The exceptions to this are the Rheometrics Melt Flow Monitor, the GUttfert Real Time Rheometer and the Brabender Auto-grader. The last is actually an in-line rheometer for monitoring the properties of feedstock (thermoplastic pellets or powders). The other two instruments are true in-line instruments in that they can be inserted in the process lines. Sidestreams are pumped through capillaries and returned to the main process stream. While these devices are capable of operating at more than one shear rate, however, pump speeds must be changed for each new shear rate. Other instruments determine either the elongational viscosity or the dynamic viscosity.
For most applications, the steady shear viscosity is the most useful measurement with precision process control and product monitoring potentially benefiting from the in-line measurement of viscosity over a wide range of shear rates. However, an adequate characterization of a thermoplastic can easily involve obtaining data over at least three decades of shear rates with several (at least five) points per decade.
In contrast, a typical on-line measurement of pressure drop and flow rate produces a single viscosity value at a single shear rate. With current technology, increasing the number of data points requires changing the flow rate by implementing auxiliary pumping capacity and acquiring data over a longer time or by cascading a series of capillaries, which still limits the number of data points to the number of capillaries.
Existing rheometers that are based upon fundamental principles and which perform measurements independent of a particular constitutive mode require many measurements in order to obtain similar information. For example, tube rheometers require that a series of flow rate measurements be made at different pressure gradients. This is very time consuming and completely impractical as an on-line process monitoring tool to measure the entire viscosity-shear rate curve. Similar limitations apply to rotational rheometers which require that torque be measured for various rotational speeds. Even the most up-to-date systems meet only the minimal requirements currently being advanced by researchers to characterize, for example, the molecular weight distribution of a thermoplastic.
The present invention overcomes the foregoing deficiencies in rheometers and rheological processing methods heretofore developed. Data can be obtained over a wide range of shear rates from a single measurement (flow rate or pressure drop). Further the invention can be applied to opaque as well as transparent systems, and hence can be used in applications ranging from thermoplastics to high density suspensions.